@droftarts, @Phaedrux, @engikeneer, thank you all for your replies. Turned out to be a faulty heater cartridge. I received a new heater cartridge from an official E3D reseller. It has the correct resistance and is working well.
I suspect the original 30w heater was faulty from the start. When I first got my V6 3 years ago, I think I was only ever able to tune to around 260 and now that I'm printing close to that, it wasn't stable while also pushing filament through.
Also, there is adequate space between the heatsink and heater block.
Glad to have my printer back. Thanks for the help!
